12.1 Toxic or Poor Quality Feed
12.2 Water Quality
12.3 Effluent
12.4 Disease
If you have not used feed on your farm before you are bound to ask this question. If you have read through the manual up to this point, most of the likely problems will have already become obvious. Many topics are not exactly problems - it is more that feeding is obviously going to create a lot more work for you and for those who work for you, especially if you decide to make your own feed. However, there will be economic benefits arising from the intensification of your farm to offset this.
I hope that this manual will have stimulated your interest in increasing the productivity of your farm through feeding and that it will have demonstrated that many other farmers find the extra effort it entails profitable. The extra work that feeding causes, if carried out properly, will increase the revenue and profit from your farm. Making your own feed should also increase your margin of profit over the use of commercial feeds, assuming that the quality of your farm-made feed is similar to that which you could buy. Feed ingredients will cost you more than the feed manufacturer, however, because you will not have his bulk buying power.
You may have extra problems to deal with once you start using feed in your farm but most of them will only occur if the feed is not prepared or applied correctly. This section of the manual is a summary of the most likely problems which could occur.
Toxic feed, which will cause growth depression and possibly mortalities, can occur if you are not careful enough to apply the advice given in this manual. In particular, feeds may be toxic if your quality control of bought-in ingredients or compound feeds is not good enough. The importance of quality control has been covered in section 5. Storage requirements have been dealt with in section 7, while some of the toxic substances which may occur in feeds are listed in Appendix XV. The topic of nutritional fish pathology has been covered in another ADCP manual (Tacon, 1985). Poor results with a specific feed may not necessarily be caused by toxicity, but simply by poor formulation (unsuitable feed) or by old feed (deteriorated quality, such as rancidity or loss of vitamin potency).
ACTION If feed is suspected to be of poor quality, stop using that batch and try another. If the problem resolves itself you will know that the feed was at fault and you will be able to investigate the cause. If the problem continues it is either caused by some aspect of the feed which is common to both batches or it is not related to the feed at all. Before you take your animals off feed, carefully consider what other things might be causing the problem you are experiencing. It is very easy to blame the feed first but it may be unfair.
The most common problem results from over-feeding, which will eventually cause water pollution in your production unit. Large quantities of excess feed build up on the bottom of the pond or tank or below cages and can result in anaerobic conditions and the production of hydrogen sulphide. Uneaten feed in the water creates an oxygen demand which may reduce dissolved oxygen levels below those necessary for the survival of the species you are culturing. Excess feed also acts as a fertilizer and may cause a plankton bloom with a similar effect on dissolved oxygen levels, especially in the early hours of the morning.
ACTION
If water quality is impaired by excess feeding you must stop feeding at once and change the water in the pond or tank as quickly as possible. The pond should be flushed to reduce the plankton density and remove toxic wastes. Feeding should re-start gradually, based on a re-assessment of biomass and feeding rate and should be more carefully monitored in future.
Feeding is only one aspect of a more intensive approach to farm management. High stocking rates and the use of feed will also increase demands on water supply for exchange purposes. It also will result in the regular discharge of soluble nutrients (waste feed and faecal matter) and an increase in the level of suspended solids during pond drainage for harvesting or other purposes. The nutrients in the effluent of the farm can, if the farm is a freshwater farm, enhance the value of the water for irrigation purposes (act as a liquid fertilizer) although the increase in suspended solids may make it unacceptable for certain types of automatic irrigation systems. Where the farm effluent is discharged into bodies of water where natural exchange is poor it may result in local plankton blooms which may trouble an ecological lobby. Some intensive salmonid farms now use feeds with reduced phosphorous content to minimize this effect. Legislation in some countries forces intensive farms to treat their effluent in the same way as an industrial user of water, so that it complies with certain standards before discharge. These may include a settlement pond and some form of biological filtration. Some farms grow other species in their outlet channels, such as molluscs or tilapia, which remove some of the nutrients while improving the quality of the effluent.
ACTION
Normally no action is necessary unless it is requested by the government body which controls the water or land into which the effluent is discharged. If you do not comply with their regulations or cause a problem for other users of that water or land, you will be advised by them what action you should take.
Diseases of fish can be transferred through the use of fresh, unpasteurized residues of the same or similar species. The use of unpasteurized trash fish as a feed for salmonids has caused this type of problem; fish or fish viscera intended for hatchery feeds is routinely pasteurizied in some countries. I have not seen reports of this type of problem occurring through the use of prawn wastes in shrimp feeds, but it is a possibility. The danger inherent in the use of raw prawn wastes is exampled by Lightner (1984). In his experiments, feeding chopped carcasses of suspect carrier shrimp to other shrimp resulted in signs of IHHN disease within 15-60 days. Pathogenic bacteria, e.g., Salmonella spp., can also be transferred in animal feed and are sometimes passed on to humans.
ACTION
If a disease problem which causes mortality or disfigures the product occurs, you must recruit the help of the local Fisheries Department. They may be able to identify the causative agent themselves or they may be able to enlist the help of specialized pathology departments such as those at the National Inland Fisheries Institute in Thailand, the University of Arizona in the USA and the Institute of Aquaculture in Scotland. If the feed is proved to be the culprit it will be necessary to change the formula or the processing technique.
